1. Field of the Invention
The invention is directed to a solid electrolyte sensor device for measuring gas compositions, which device has a solid electrolyte arranged between at least two electrodes. The electrodes are gas permeable and at least one of the electrodes faces the measurement gas. The device also has an electrical device for detecting and processing concentration dependent sensor output values.
2. Description of the Prior Art
Solid electrolyte sensors of the type mentioned above, which are used to determine gas concentrations, are galvanic chains formed by a solid electrolyte and two or more electrodes. The solid electrolyte itself is a solid body which is conductive for at least one type of ion due to its imperfection. In this context, imperfection is understood to mean the divergence of a crystal lattice from the specific ideal orderly structure. At final temperatures, there may be unoccupied positions in a crystal lattice- so-called vacancies and additional interstitial atoms or ions in the intersticies, i.e. between the regular positions of the host lattice. Other imperfections may consist of foreign ions located, for example, on interstitial sites. The ion conductivity depends on the temperature and increases as the temperature increases. The basic use of such ion conductors as solid electrolyte sensors is known from the article "Solid Ion Conductors-Fundamentals and Applications" (Prof. Dr. Rickert), Applied Chemistry 90, 38 to 48 (1978). A very popular example of a solid electrolyte sensor is the so-called lambda probe for determining oxygen concentrations in motor vehicle exhaust, waste gases from furnace installations, and the like.
Solid electrolyte sensors are known for verifying or determining the concentration of other gas components such as chlorine, sulfur dioxide, carbon dioxide, nitric oxides, etc. In these solid electrolyte sensors, the solid electrolyte does not convert the gas component to be verified- as with zirconium dioxide and oxygen - but rather conducts ions which react with these gas components to form a chemical bond. In many applications, verification of the gas component by a solid electrolyte requires conversion of the gas component to be measured. This is true, for example, when measuring sulfur dioxide which must be converted to sulfur trioxide. Platinum or vanadium pentoxide catalysts are conventionally used for this purpose.
In general, the problem with known arrangements is that the measurement gas component can only be verified with difficulty, if at all, or that the measurement gas component overreacts chemically with the electrode material and is adsorbed on the measurement electrode. It is known to use the above-mentioned catalysts to prevent such interaction with the measurement electrode. However, these catalysts bring about a pure surface reaction with the measurement gas. This has the grave disadvantage that the catalytic efficiency is not constant and accordingly constant measurement conditions cannot be maintained along the time axis. Regulated heating of such catalysts allows the catalyst function to be regulated to a certain extent, but is impossible when using gas sensors having a sensitive reaction to gas compositions and fluctuations in gas compositions. This is because very high catalyst temperatures are employed almost exclusively so that considerable changes are brought about in the measurement gas due to thermal induction of chemical processes. Consequently, such catalysts cannot be used in sensitive gas sensors as a rule. Also, for the most part, catalysts are not available for every gas component to be measured. Apart from a purposeful oxidation of the measurement gas, there remains the critical problem of the reactivity of the measurement gas with the measurement electrode.
Solid electrolyte sensors often have cross-sensitivity to gases with high absorption at the measuring electrode. For example, chlorine sensors exhibit a substantial cross-sensitivity to hydrogen sulfide because the sulfur split off from the hydrogen sulfide coats the measurement electrode of the solid electrolyte sensor. Reductive gases such as hydrogen interfere with the measuring properties of solid electrolyte sensors, since these gases reduce the oxidic solid electrolyte and accordingly permanently change its material characteristics. Naturally, in time, this not only leads to a reduction in measuring accuracy, but also to the destruction of the sensor.
When measurement gas components and ions of the solid electrolyte form oxidic compounds, the electromotive force generated by the sensor also depends on the concentration of oxygen. This means that a knowledge of the oxygen concentration is necessary for determining the concentration of the measurement gas component. An additional oxygen sensor is accordingly required. This is a disadvantage in DE-OS 36 33740, in which precisely such an oxygen sensor is necessary.